A retrospective long-term assessment of antibody response in melioidosis animals

Melioidosis is an extremely fatal infection disease caused by the gram-negative bacterium Burkholderia pseudomallei. The causative agent of melioidosis is an environmental saprophyte and has significant adaptive potential allowing it to undergo rapid adaptation to a wide variety of ecological niches. The bacterium is a facultative intracellular pathogen that can infect many animal species as well as humans. B. pseudomallei has no pronounced tropism for certain tissues of the infected organism, which explains the diversity and non-specificity of clinical manifestations, the most common of which are pneumonia with or without bacteremia and sepsis. Тhis diagnosis is established on the basis of laboratory test data; cultured pathogen isolation is considered the gold standard. The indirect hemagglutination test remains a popular method for rapid melioidosis diagnostics, and increased paired serum antibody titer is an important criterion for making a diagnosis. The purpose of this work was to conduct a retrospective analysis of the long-term dynamic antibody response in Asiatic black bears that died from melioidosis and contact animals from the Vietnam Bear Rescue Center as well as molecular genetic analysis of isolated B. pseudomallei strains. Bacterial cultures were isolated from dead animals and identified by PCR as B. pseudomallei. Multilocus sequence typing showed that strains VP069 and VP044 had the same and hitherto undescribed sequence type (STnew), and strains VP161 and VP200 shared ST541. The pairwise strain identity with the same ST was confirmed by a core genome multilocus sequence typing. A retrospective analysis of 546 serum samples from 226 bears was carried out. All animals that died from melioidosis were seropositive for B. pseudomallei. Among the healthy bears studied, 24% were seropositive. It was found that in humans and bears there is a general trend in the dynamics of the antibody response. Similar to humans, bears with bacteriologically confirmed melioidosis were observed to have both higher and lower serum antibody titer, as well as a constant and fairly high titer, which shows the diagnostic ineffectiveness elevated paired serum antibody titer in melioidosis.